Method of inhibiting radioactive substances from eluting into cooling water in a nuclear plant and an apparatus therefor

ABSTRACT

Disclosed is a method and apparatus for inhibiting radioactive substances eluting into cooling water of a nuclear plant. The method uses an index consisting of an amount of iron adhered onto the fuel cladding surface, that is calculated from the iron concentration of the cooling water and the operation time. A formation of a layer of the iron oxide on the fuel cladding surface is confirmed based upon the covering ratio of 100%. When the covering ratio is smaller than 100%, the iron concentration in the cooling water is controlled to remain constant at a maximum concentration. The Fe/Ni molar concentration ratio in the cooling water is adjusted to be set from about 2 to 10 after the layer of iron oxide reaches a covering ratio of 100%.  58  Co ion and  60  Co ion concentrations in the cooling water can be decreased without greatly increasing the concentration of precipitating radioactive crud, and the surface dosage in the primary system can be decreased at the time of regular checking.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a method of inhibiting radioactivesubstances from eluting into the cooling water in a nuclear plant, suchas a boiling water reactor (BWR) or an advanced converter, and to anapparatus therefor. More particularly, the present invention relates toa method of inhibiting radioactive substances from eluting into thecooling water in a nuclear plant in which the iron concentration in thecooling water is reduced smaller than 1 ppb, wherein the surface dosagein the primary system is reduced, and to an apparatus therefor.

2. Description of the Prior Art

A conventional method of inhibiting radioactive substances from elutinginto the cooling water in the boiling water reactor has been disclosed,for example in Japanese Patent Laid-Open No. 79194/1986, according towhich, when the boiling water reactor is placed in the businessoperation, the Fe/Ni molar concentration ratio in the boiling waterreactor is controlled at a level of 2 to 8; however, no satisfactoryresults are produced therein.

The above conventional method is practiced under the observation bymodel analysis that the minimum surface dosage in the primary system isobtained after about 3000 hours from the start of operation of theboiling reactor. Therefore, the control of the Fe/Ni molar concentrationratio is practiced inevitablely after about 3000 hours from thebeginning of the operation of the boiling water reactor.

Besides in another prior art method of inhibiting radioactive substancesfrom eluting into the cooling water in the boiling water reactor, theiron concentration in the cooling water is gradually increased with theradioactive cobalt (⁵⁸ Co) ion concentration in the cooling water as anindex, nickel ions being brought into the nuclear reactor from the feedwater system becoming the radioactive cobalt (⁵⁸ Co) ions.

In the above mentioned conventional method, however, no considerationhas been given to the iron crud, for example α-Fe₂ O₃, on the fuelcladding surface, and to the covering ratio at which the outer surfaceof the fuel rod including the fuel pellets therein, or the outer surfaceof the fuel cladding tube, is covered with the iron crud that affectsthe reaction of the iron crud with nickel and cobalt. The corrosionsubstance formed on the cladding tube is iron, cobalt or nickel, theradioactive substances being ⁵⁸ Co, ⁶⁰ Co or ⁵⁴ Mn.

During the initial stage of the operation cycle in the businessoperation and in case the nickel ion concentration has dropped greatly,furthermore, the index, i.e., the Fe/Ni molar concentration ratio,becomes no more effective in decreasing the ⁵⁸ Co ion and ⁶⁰ Co ionconcentrations in the cooling water. Further, when it is attempted toincrease the Fe/Ni molar concentration ratio, the iron crud may beintroduced in an excess amount, whereby ⁵⁴ Mn ions formed by thereaction of ⁵⁴ Fe(n, p) often causes the surface dosage in the primarysystem to increase.

When the Fe/Ni molar concentration ratio on the fuel cladding surfaceincreases, the activity of ⁵⁴ Mn crud or ⁶⁰ Co crud increasesqualitatively and, however, ⁶⁰ Co ion or ⁵⁸ Co ion decreasesqualitatively.

An object of the present invention is to provide a method of inhibitingradioactive substances from eluting into the cooling water in a nuclearplant, wherein the iron concentration in the cooling water can besuppressed, and the amount of sedimenting crud-like (insoluble)radioactive substances is as small as possible.

Another object of the present invention is to provide a method ofinhibiting radioactive substances from eluting into the cooling water ina nuclear plant, wherein the iron concentration in the cooling water canbe controlled to decrease the ⁵⁸ Co ion and ⁶⁰ Co ion concentrations inthe cooling water.

A further object of the present invention is to provide a method ofinhibiting radioactive substances from eluting into the cooling water ina nuclear plant, wherein the surface dosage in the primary system can bedecreased at the time of regular checking contributing to decreasing thesurface dosage in the primary system to which a worker may be exposed.In the present invention, the "crud" is defined as a particle substancewhich does not pass through the myriapore filter having 0.45 μm pore.The "ion" is defined as a substance which passes through the myriaporfilter having 0.45 μm pore.

The above-mentioned objects are achieved by a method of inhibitingradioactive substances from eluting into the cooling water in a nuclearplant in which after a new fuel cladding, which is an unused one and isloaded into the nuclear reactor for the first time and further does nothave material substance adhering on the fuel cladding surface, is loadedinto the nuclear reactor. With nuclear heating, an iron ion source isinjected into the cooling water in order to form an iron oxide layer onthe fuel cladding surface; and after the iron oxide layer is formed onthe fuel cladding surface at a covering ratio of 100%, an injectingamount of the iron ion source is lowered so that an Fe/Ni molarconcentration ratio in the cooling water is set in a range of from about2 to 10.

The iron ion or the iron crud is injected during the pre-operation toform the iron oxide layer on the fuel cladding surface at the coveringratio 100%. The iron concentration in the cooling water is controlled bycontrolling a flow rate of the cooling water through a condensed waterby-pass line. The iron concentration in the cooling water is controlledby controlling an iron electrolyzing current.

Monitoring the amounts of iron, nickel and cobalt in the cooling waterof the boiling water reactor makes it possible to learn the amounts ofcorrosion products such as iron crud, nickel ion and cobalt ion that arebrought into the nuclear reactor within a period of time in whichmeasurement is taken, and serves as an input that is necessary forcalculating the amount of material substance adhered on the fuelcladding surface.

Further, estimating the amount of material substances adhered on thefuel cladding surface by calculation, and controlling the amountthereof, are effective for efficiently reacting the iron crud thatbuilds up on the fuel cladding surface with nickel and cobalt, forreducing the ⁵⁸ Co ion and ⁶⁰ Co ion concentrations in the coolingwater, and for inhibiting the amount of undesired radioactive crud suchas ⁵⁴ Mn from increasing. The amount of the iron crud adhered on thefuel cladding surface can be decreased by controlling the ionconcentration of the iron crud in the cooling water.

In accordance with the present invention, in an apparatus for inhibitingradioactive substances from eluting into the cooling water in a nuclearplant, the nuclear plant includes a nuclear reactor, a steam turbine, acondenser, a condensed water purifying unit, a feed water heater, arecirculation system, a means for measuring iron concentration in thecooling water on a downstream side of the feed water heater, a means formeasuring nickel concentration in the cooling water on the downstreamside of the feed water heater, a means for injecting iron into thecooling water on the downstream side of the condensed water purifyingunit, and a control unit for controlling the iron concentrationinjecting means so as to adjust an amount of iron injected based upon asignal sent from the iron concentration measuring means and the nickelconcentration measuring means and for representing an amount of the ironconcentration.

A means for controlling the iron concentration in the cooling water isprovided in the nuclear plant; the iron concentration controlling meanscomprise a means for evaluating a total amount of iron concentrationadhered on the fuel cladding surface based upon the iron concentrationin the cooling water, a means for controlling the amount of ironinjected into the cooling water based upon the total amount of the ironconcentration on the fuel cladding surface, and a means for supplyingiron that is to be injected through the iron concentration injectingamount controlling means.

According to the present invention, ⁵⁸ Co ion and ⁶⁰ Co ionconcentrations in the cooling water can be decreased without muchincreasing the concentration of precipitating radioactive crud such as⁶⁰ CoFe₂ O₄, ⁵⁸ CoFe₂ O₄ or ⁵⁴ MnFe₂ O₄, and the surface dosage in theprimary system can be decreased at the time of regular checkingcontributing to decreasing the surface dosage in the primary system towhich a worker may be exposed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a feed water system in a boiling waterreactor according to one embodiment of the present invention;

FIG. 2 is a diagram which schematically illustrates the fuel claddingsurface on an enlarged scale;

FIG. 3 is a flow chart which illustrates a method of controlling theiron concentration of the cooling water according to the presentinvention;

FIG. 4 is a diagram which illustrates a pattern for controlling the ironconcentration of the cooling water according to the present invention incomparison with that of the conventional method;

FIG. 5 is a diagram which quantitatively illustrates the effectsaccording to one embodiment of the present invention;

FIG. 6 is a diagram illustrating one structure of the nuclear plant whenthe present invention is adapted to a practical nuclear plant system;and

FIG. 7 is a diagram illustrating another structure of the nuclear plantwhen the present invention is adapted to a practical nuclear plantsystem.

DESCRIPTION OF THE EMBODIMENTS

Embodiment 1:

One embodiment of the present invention will now be described inconjunction with FIG. 1 which illustrates a feed water system of aboiling-water reactor. The steam generated from a steam turbine 1 is ledinto a condenser 2. The condensed water from the condenser 2 containscorrosion products in large amounts.

However, most of the corrosion products such as iron, cobalt and nickelis removed as the condensed water that is pumped by a pump 3 passesthrough a condensed water pre-filter 4 and a condensed water desaltingunit 5. The purified water passes through a water supply pump 8, alow-pressure feed water heater 9, a booster pump 10, a high-pressurefeed water heater 11, and is guided to a pressurized vessel 15 of thenuclear reactor.

The corrosion products brought into the pressurized vessel 15 of thereactor consist of those that were not removed by the condensed waterdesalting unit 5 and nickel etc. that is generated chiefly by thecorrosion of the high-pressure feed water heater 11. The amount broughtin can be detected by measuring the sampling water using a concentrationmeasuring device 13 for measuring an iron concentration, a nickelconcentration, and an cobalt concentration; the sampling water isobtained through a sampling line 12.

According to one embodiment of the present invention, after the new fuelrod is loaded into the nuclear reactor, pre-operation in the boilingwater reactor is carried out. With the nuclear heating the iron ionsource such as an iron crud or an iron ion is injected into the coolingwater in order to form the iron oxide layer on the fuel claddingsurface.

According to one embodiment of the present invention, the measuredconcentration value is guided to a controller 14 to estimate the amountof adhesion of the substance such as the iron crud on the fuel claddingsurface, and a valve 7 is so adjusted that the iron is injected from aniron injecting device 6 into the cooling water at an optimum ironconcentration.

The reasons for estimating and controlling the amount of materialsubstance adhering on the fuel cladding surface will be described inconjunction with FIG. 2, which schematically illustrates a fuel claddingsurface 16 on an enlarged scale and which represents the case where theiron is adhered in small amount on the fuel cladding surface 16. Theiron adheres on the fuel cladding surface 16 mostly in the form of ironcrud particles 17.

If the fuel cladding surface 16 is not sufficiently covered with theiron crud particles 17, a layer 18 of NiO or CoO is formed on the fuelcladding surface 16. The layer 18 of NiO or CoO serves as a main factorthat raises the ⁵⁸ Co ion and ⁶⁰ Co ion concentrations in the coolingwater.

Therefore, the fuel cladding tube or the fuel rod that is covered withthe iron crud particles 17 with a covering ratio 100% is effective ininhibiting the formation of the NiO layer 18 or CoO layer 18. When theiron crud particles 17 adhere on the fuel cladding surface 16 in anexcess amount, on the other hand, the iron crud concentration in thecooling water rises when the iron crud particles 17 are peeled offeasily.

The iron crud particles 17 then serve as carriers, and the precipitatingradioactive crud such as ⁶⁰ CoFe₂ O₄, ⁵⁸ CoFe₂ O₄ or ⁵⁴ MnFe₂ O₄ resultsin an increase in the surface dosage in the primary system of pedestalunder the pressurized vessel 15 of the nuclear reactor.

Therefore, the amount of the iron crud particles 17 adhered on the fuelcladding surface 16 should desirably be as small as possible, but besufficient for covering the surface of the fuel cladding tube or thefuel rod and be also sufficient to react with nickel or cobalt to formNiFe₂ O₄ or CoFe₂ O₄.

The amount of the iron adhered on the fuel cladding surface 16 can becontrolled by controlling the amount of the iron brought in by the feedwater, i.e., by controlling the iron concentration in the cooling water.The control method will now be explained in conjunction with FIG. 3which is a flow chart for controlling the iron concentration in the feedwater.

Concentrations of iron, nickel and cobalt in the cooling water aredenoted by CFe, CNi and CCo, measuring interval is denoted by t, flowrate of the cooling water is denoted by F, and the amounts of iron,nickel and cobalt which adhere onto the fuel cladding surface aredenoted by MFe, MNi and MCo.

Then, the amounts MFe', MNi' and MCo' of iron, nickel and cobalt whichnewly adhere onto the fuel cladding surface can be estimated as follows:

    MFe'=MFe+aCFeFt-ζcMFet

    MNi'=MNi+bCNiFt-ζiMFet

    MCo'=MCo+cCNiFt-ζiMCot

where a, b and c are correction coefficients for estimating the amountthat newly adhere to the fuel cladding surface from the concentration ofthe feed water, and where a has a value of from 0.7 to 1.0, and b and chave values of from 0.6 to 1.2 that vary depending upon the constitutedmaterials of the nuclear plant. Further, symbols ζc and ζi correspond torate coefficients that decrease due to peeling off or elution of thecrud or the ion from the fuel cladding surface.

A setpoint value d of the amount of iron adhered onto the fuel claddingsurface is found from the following equation ##EQU1## where the ironcrud layer has a thickness e of from 0.2 to 1.0 μm, so that the fuelcladding surface is sufficiently covered with iron, the total outersurface area of the fuel cladding tube is denoted by S and the hematite(Fe₂ O₃) has a density of 5.2 g/cm³.

Here, if the total outer surface area of the fuel cladding tube S=7×10⁷cm and the thickness of the iron crud layer e=1.0 μm, then the amount ofthe iron adhered onto the fuel cladding surface setpoint value d becomesabout 25 Kg. When the iron concentration amount MFe does not reach thesetpoint value d of the amount of the iron adhered onto the fuelcladding surface that is determined as described above, the ironconcentration in the cooling water is adjusted to be from about 0.3 to 1ppb so that the iron concentration amount MFe exceeds the setpoint valued as quickly as possible.

Here, however, if the iron concentration in the cooling water is soincreased as to exceed 1 ppb, the iron crud may undesirably build up onportions other than the fuel cladding tube. When a relationship MFe>dsatisfied, a relationship MFe>2(MNi+MCo) will be maintained bycontrolling the iron concentration in the cooling water, so that theiron will not be brought into the nuclear reactor in an excess amount.

The iron concentration in the cooling water can be measured by a methodaccording to which the cooling water is permitted to pass through amyriapore filter. The nickel and cobalt concentrations in the coolingwater can be measured by a method according to ion exchange resin thatis now widely used to trap and condense them followed by analysis by theatomic absorption method, or can be measured by a method which uses aconductivity measuring instrument in an on-line manner. When measuredbatchwise, the results of measurement must be input to the controller 14in an off-line manner.

FIG. 4 shows the change of iron concentration X1 in the cooling waterwith the passage of time when the present invention is adapted incomparison with when a conventional method X3, X5 is employed.

According to the conventional method, the iron concentration in thecooling water is gradually increased with the radioactive cobalt (⁵⁸ Co) ion concentration in the cooling water as an index, the nickel ionsbeing brought into the nuclear reactor from the feed water system becomethe radioactive radioactive cobalt (⁵⁸ Co ) ions.

According to one embodiment of the present invention, on the other hand,the index consists of an amount of iron adhered onto the fuel claddingsurface that is calculated from the iron concentration of the coolingwater and the operation time.

In FIG. 4, furthermore, a covering ratio (%) ×4 found from the amount(W) of the iron adhered on the fuel cladding surface and a setpointamount (W₀) of the iron adhered on the fuel cladding surface inaccordance with the following equation, is indicated as an index, i.e.,##EQU2##

That is, according to one embodiment of the present invention,concentration (C) of iron ion or iron crud in the cooling water ismeasured, a covering ratio (%) of an iron oxide film or an iron oxidelayer on the fuel cladding surface is found based upon the heat flux (Q)of the fuel rod or the fuel cladding tube, latent heat (L) ofvaporization, the operation time (t), and a deposition rate coefficient(K) in accordance with the following equation (i) and (ii):

    W=K·Q·C·t/L                     (i)

    covering ratio (%)=(W/W.sub.0)×100                   (ii)

The formation of the layer of the iron oxide on the fuel claddingsurface is confirmed based upon the covering ratio of 100%, and theoxidation processing is finished.

When the covering ratio is smaller than 100% in the present invention,the iron concentration in the cooling water is controlled to remainconstant, which is maximum concentration permitted under the operationcondition of the nuclear plant (corresponds to a period A in FIG. 4).

Desirably, the Fe/Ni molar concentration ratio of the cooling water iscontrolled to become about 2 that is necessary for forming NiFe₂ O₄ at amoment when the covering ratio has reached 100%.

FIG. 5 is a diagram in which the change of ⁵⁸ Co ion concentration inthe cooling water with the passage of time is analytically found at thetime when the iron concentration of the cooling water is controlledaccording to two kinds of patterns shown in FIG. 4.

According to the conventional method Y2, as will be obvious from FIG. 5,the overshoot phenomenon develops in the ⁵⁸ Co ion concentration.According to embodiment Y1 of the present invention, on the other hand,the overshoot phenomenon developing in the ⁵⁸ Co ion concentration isprevented from taking place. Consequently, therefore, the radioactivesubstances build up in least amounts on the inner surfaces of conduitsabout the reactor core.

According to one embodiment of the present invention, the Fe/Ni molarconcentration ratio in the cooling water is adjusted to be set fromabout 2 to 10 after the layer of iron oxide has been formed under theoperation condition of the nuclear plant (corresponds to a period B inFIG. 4). This is because when the Fe/Ni molar concentration ratio isgreater than 2, the nickel ions and cobalt ions adhered on the fuelcladding surface can exist in the form of a compound oxide at a rate of1 to 2 with respect to the iron, i.e., can exist in the form of NiFe₂ O₄and CoFe₂ O₄.

When the Fe/Ni molar concentration ratio is smaller than 2, however, thenickel ions and cobalt ions are present in an excess amount with respectto the iron, whereby the nickel ions and cobalt ions tend to exist assole oxides, i.e., as NiO or CoO.

When the Fe/Ni molar concentration ratio is zero, the nickel ions andcobalt ions all adhere in the form of NiO and CoO on the fuel claddingsurface. When the Fe/Ni molar concentration ratio is excessively high,on the other hand, the iron crud adheres in large amount on the fuelcladding surface after the nuclear reactor is operated for extendedperiods of time, and the peeling of the radioactive crud (⁶⁰ CoFe₂ O₄,⁵⁸ CoFe₂ O₄) formed on the fuel cladding surface can no more beneglected.

The same effects are obtained even when the Fe/Ni molar concentrationratio is controlled to become greater than 2. In this case, however, theiron is adhered on the fuel cladding surface in an excess amount,whereby crud-like (insoluble) radioactive cobalt is formed at aincreased rate.

According to one embodiment of the present invention, when the ironoxide layer is formed on the fuel cladding surface at the covering ratioof 100%, the Fe/Ni molar concentration ratio should particularlypreferably be to be set from about 4 to 6.

Embodiment 2:

FIG. 6 concretely illustrates a control system of when the presentinvention is adapted to a practical nuclear plant. In FIG. 6, referencenumeral 21 denotes a condensed water by-pass line, 22 denotes a by-passflow rate control valve, and 23 denotes a valve open/close controller.The condensed water by-pass line 21 is formed between the pump 3 and thecondensed water desalting unit 5 bypassing the condensed waterpre-filter 4. The by-pass flow rate control valve 22 is provided on thecondensed water by-pass line 21, and the valve open/close controller 23is connected to the by-pass flow rate valve 22.

In response to a control signal from a device 14 which evaluates theamount of the iron adhered on the fuel cladding surface, the valveopen/close controller 23 operates to change the opening degree of theby-pass flow rate control valve 22, so that the flow rate of the coolingwater through the by-pass changes.

As the flow rate of the cooling water through the condensed waterby-pass line 21 increases, the efficiency for removing the iron cruddecreases through the condensed water pre-filter 4 and the ironconcentration in the cooling water increases.

As the flow rate of the cooling water through the condensed waterby-pass line 21 decreases, on the other hand, the iron concentration ofthe cooling water decreases. By controlling the flow rate of the coolingwater through the condensed water by-pass line 21, therefore, the ironconcentration in the cooling water can be controlled according to anoptimum pattern that is shown in FIG. 4.

Embodiment 3:

FIG. 7 concretely illustrates another control system where the presentinvention is adapted to a practical nuclear plant. In FIG. 7, referencenumeral 24 denotes a feed water by-pass line, 25 denotes an ironelectrolyzing device, 26 denotes a DC power source for electrolysis, and27 denotes an electrolyzing current controller.

The feed water by-pass line 24 is formed between the condensed waterdesalting unit 5 and the low-pressure feed water heater 9. The ironelectrolyzing device 25 is provided at the downstream of the feed waterby-pass line 24. The DC power source 26 for electrolysis is connected tothe iron electrolyzing device 25. The electrolyzing current controller27 is connected to the DC power source 26. In response to a controlsignal from the device 14 which evaluates the amount of the ironconcentration adhered on the fuel cladding surface, the electrolyzingcurrent controller 27 operates to change the electrolyzing current ofthe iron electrolyzing device 25. The amount of the iron ions generatedin the iron electrolyzing device 25 increases with the increase in theelectrolyzing current, whereby the iron concentration of the coolingwater increases.

On the contrary, the amount of the iron ions decreases with the decreasein the electrolyzing current, whereby the iron concentration in thecooling water decreases. By controlling the iron electrolyzing current,therefore, the iron concentration in the cooling water can be controlledin accordance with an optimum pattern that is shown in FIG. 4.

Modified Embodiments:

In the aforementioned embodiments of the present invention, the totalamount of iron that is necessary to completely cover the whole fuelcladding surface with the iron crud was used as an index for controllingthe iron concentration in the cooling water. It is, however, allowableto use another index which represents the adhesion of the iron crudformed on the fuel cladding surface. For example, it is allowable to usethe total amount of the iron crud adhered on the fuel cladding surfacethat is obtained from the experience in operating the nuclear plant.

The same effects can also be obtained even when the adhesion of the ironcrud as visually observed through a periscope, or signals from a sensorthat measures the physical quantities reflecting the adhering amount ofthe iron crud on the fuel cladding surface, are used for determiningwhether the amount of the iron crud adhered on the fuel cladding surfacehas reached a predetermined or preset value or not on the flow chart ofFIG. 3. In general, however, it is difficult and lacks practicality tobring into the nuclear reactor the devices for carrying out theobservation and the measurement.

What is claimed is:
 1. A method of inhibiting radioactive substancesfrom eluting into the cooling water in a nuclear plant, wherein, after anew fuel cladding is loaded into a nuclear reactor, a source of ironions is injected into the cooling water, during nuclear heating, inorder to form an iron oxide layer on a surface of the new fuel claddingwith a covering ratio of 100%; and, after said iron oxide layer has beenformed on said surface of the new fuel cladding, an amount of saidsource of iron ions injected into the cooling water is decreased so thata Fe/Ni molar concentration ratio in the cooling water is set in a rangeof from about 2 to
 10. 2. A method of inhibiting radioactive substancesfrom eluting into the cooling water in a nuclear plant according toclaim 1, wherein a source of iron ions is injected during apre-operation of the nuclear plant to form said iron oxide layer on saidsurface of the new fuel cladding.
 3. A method of inhibiting radioactivesubstances from eluting into the cooling water in a nuclear plantaccording to claim 1, wherein said Fe/Ni molar concentration ratio iscontrolled to be about 2 when said iron oxide layer covers said surfaceof the new fuel cladding at the covering ratio of 100%.
 4. A method ofinhibiting radioactive substances from eluting into the cooling water ina nuclear plant according to claim 1, wherein the iron oxide layerformed on the surface of the new fuel cladding is the minimum necessaryto provide the covering ratio of 100% and to react with nickel or cobaltto form NiFe₂ O₄ or CoFe₂ O₄.
 5. A method of inhibiting radioactivesubstances from eluting into the cooling water in a nuclear plantaccording to claim 1, wherein the iron oxide layer has a thickness of0.2 to 1.0 μ m.
 6. A method of inhibiting radioactive substances fromeluting into the cooling water in a nuclear plant according to claim 1,wherein an iron concentration in the cooling water is controlled bycontrolling a flow rate of the cooling water through a condensed waterby-pass line.
 7. A method of inhibiting radioactive substances fromeluting into the cooling water in a nuclear plant according to claim 1,wherein an iron concentration in the cooling water is controlled bycontrolling an iron electrolyzing current.
 8. A method of inhibitingradioactive substances from eluting into the cooling water in a nuclearplant according to claim 1, wherein said source of iron ions is an ironcrud, said iron crud being injected during a pre-operation of thenuclear plant to form said iron oxide layer on said surface of the newfuel cladding.
 9. A method of inhibiting radioactive substances fromeluting into the cooling water in a nuclear plant according to claim 8,wherein the adhesion of iron crud as visually observed is used fordetermining whether an amount of said iron crud adhered on said fuelcladding surface has reached a preset value or not.
 10. A method ofinhibiting radioactive substances from eluting into the cooling water ina nuclear plant according to claim 1, wherein a concentration of saidsource of iron ions in the cooling water is measured; a covering ratio(%) of said iron oxide layer on said surface of the new fuel claddingsis found from the following equations (i) and (ii), where K is adeposition rate coefficient, Q is heat flux of the new a fuel cladding,C is concentration of source of iron ions, t is an operation time, L isa latent heat of vaporization, W is an amount of iron adhered on saidsurface of the new fuel cladding, and W_(O) is a setpoint amount of ironadhered on said surface of the new fuel cladding,

    W=K Q C t/L                                                (i)

    covering ratio (%)=(W/W.sub.0)×100;                  (ii)

formation of said iron oxide layer on said surface of the new fuelcladding is confirmed based upon the covering ratio; and an oxidationprocessing to form on the surface of the new fuel cladding the ironoxide layer is finished when the covering ratio of said iron oxide layerreaches 100%.
 11. A method of inhibiting radioactive substances fromeluting into the cooling water in a nuclear plant according to claim 10,wherein said Fe/Ni molar concentration ratio is controlled to be about 2when said iron oxide layer is covered on said surface of the new fuelcladding at the covering ratio of 100%.
 12. A method of inhibitingradioactive substances from eluting into the cooling water in a nuclearplant according to claim 10, wherein, after said iron oxide layer hasbeen formed at the covering ratio of 100%, said Fe/Ni molarconcentration ratio in the cooling water is adjusted to a value of fromabout 2 to
 10. 13. A method of inhibiting radioactive substances fromeluting into the cooling water in a nuclear plant according to claim 12,wherein said Fe/Ni molar concentration ratio is adjusted to a value offrom about 4 to 6.